The present invention is directed to an automatic conveyorized vacuum applicator and method of operation thereof having utility in the application of dry film photoresist-forming materials, such as photoresists and solder masks, to surfaces of printed circuit boards or other substrates, to ensure complete conformity of the dry films around raised circuit traces and irregular surface contours. The applicator and method have particular utility for conveying and for applying vacuum, heat, and mechanical pressure to printed circuit boards or other substrates that prior to such application have had dry film loosely applied to at least one of the surfaces thereof as discrete cut sheets within the confines of the substrate.
Primary imaging photoresists and secondary imaging solder masks are among the most widely used photoimageable materials in the manufacture of printed circuit boards. A primary imaging photoresist is used in the creation of the printed circuitry itself, whereas a solder mask is used to protect the printed circuitry during soldering of components onto the board.
A primary photoresist is a hard, temporary layer of non-conductive material which covers the metal surface of a copper-clad substrate that later becomes the printed circuit board. The photoresist is patterned in such a way so as to produce a resist stencil around which the printed circuit tracks are formed.
More specifically, primary photoresists, typically,are formed from a layer of photoimageable composition which is applied to the surface of a copper-clad board. The photoimageable composition is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent, aqueous, or semi-aqueous solution which washes away either exposed or unexposed portions of the layer (depending on whether the photoimageable material is positive-acting or negative-acting). Thereafter, the circuit traces are formed by either electroplating or etching. In a typical plating procedure, the areas devoid of photoresist that become the circuitry are built up from the board surface by electroplating copper thereon. After protecting the electroplated copper layer, the remaining photoresist is stripped away in an organic solvent, aqueous, or semi-aqueous solution, and the newly exposed areas of metal are then selectively removed in an etching solution, leaving behind the pattern plated copper circuit lines. In a typical etching procedure, the metal in the areas devoid of photoresist is selectively removed in an etching solution, leaving behind the residual portions of the etched metal layer as the circuit traces after the primary resist is stripped away.
A solder mask, on the other hand, is a hard, permanent layer of non-conductive material which covers the surface of a printed circuit board or other substrate, encapsulating the traces of the printed circuitry itself. The solder mask is patterned to fully cover the circuitry, except for those portions intended to be exposed, e.g., for soldering to another component.
More specifically, solder masks, typically, are formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. Similar to primary imaging resists, the photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent, aqueous, or semi-aqueous solution which washes away either exposed or unexposed portions of the layer (again depending upon whether the photoimageable material is positive-acting or negative-acting). The portion of the layer which remains on the surface is then cured, e.g., with heat and/or UV light, to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board.
One prior art method of applying a layer of primary resist or solder mask to a circuit board surface is to apply the material in liquid form, and then, either allow it to dry or partially cure the material to form a semi-stable layer. There are a number of advantages, however, to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions.
Typically, such a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the cover sheet. The cover sheet may be formed of polyester material, such a polyethylene terephthalate (PET). To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional for the exposed surface of the photoimageable layer to be covered with a removable protective sheet, e.g., a sheet of polyethylene.
The method of use of such a dry film is generally as follows. The protective polyethylene sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Then using either heat and mechanical pressure (in the case of roll laminators) or a combination of vacuum, heat, and mechanical pressure (in the case of vacuum laminators), the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits a pattern (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET cover sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer. The photoimageable layer may either be positive-acting, in which case the exposed portions are removed by developer, or negative-acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing primary imaging photoresists and solder masks are negative-acting. Subsequent to development, primary resists, in particular, are subjected to either electroplating or etching, as previously described, to form the circuit traces after which the remaining photoresist is stripped away with organic solvent, aqueous stripper, or semi-aqueous stripper. Whereas, in the case of solder masks which remain on the board permanently, most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light.
Printed circuit boards almost invariably have uneven surfaces which present difficulties for dry film application. During solder mask application, in particular, such unevenness is usually attributed to the circuitry traces which are raised or elevated over the surface of the board of electrically non-conducting material. It is therefore desirable that any dry film solder mask applied to the board be able to conform around the upstanding circuitry traces to minimize the risk of defects, such as short circuits. On the other hand, during primary resist application, such unevenness usually arises when creating circuitry on thin outer surfaces of multi-layered circuit boards which contain embedded components that protrude and leave impressions on the outer surface. It is desirable that any photoresist applied to such a board be able to conform to such irregular surface contours to minimize the formation of defects, such as voids, disconnects, or shorts. There has also been a demand on circuit board manufactures, due to the current trend to miniaturize electronic equipment, to reduce the size of printed circuit boards while increasing their functional capabilities which presents other difficulties for dry film photoresist application. As more circuitry needs to be fit onto smaller surfaces, the circuit lines and spaces therebetween on the circuit board have continued to shrink. The creation of this fine line and closely spaced circuitry can be achieved only with difficulty and only if the primary resist fully adheres and completely conforms to the contours of the printed circuit board. Otherwise, voiding of the minute circuit traces and creation of disconnects or shorts will occur.
A number of improved photoimageable dry films and vacuum lamination processes have been developed to try to improve the conformance of the dry film to the irregular surface contours of a printed circuit board, as for example, as disclosed in U.S. Pat. No. 4,889,790 (Roos et al.), U.S. Pat. No. 4,992,354 (Axon et al.), and U.S. Pat. No. 5,164,284 (Briguglio et al.), The processes disclosed in these patents involve applying a photoresist-forming layer to a printed circuit board using a dry film in which an xe2x80x9cintermediate layerxe2x80x9d selected for its transparency, strength and flexibility is interposed between the support film or cover sheet and the photoimageable layer. The intermediate layer of the dry film is selectively more adherent to the photoimageable composition layer than to the cover sheet, allowing the cover sheet to be removed after the photoimageable layer is laminated to a printed circuit board to assist conformance, with the intermediate layer remaining on the photoimageable composition layer as a xe2x80x9ctop coat.xe2x80x9d The top coat is of non-tacky material and can be placed in contact with other surfaces, such as artwork for contact printing. The top coat also serves as an oxygen barrier, allowing the photoimageable composition layer to remain unexposed on the printed circuit board, after cover sheet removal, for some length of time. The use of dry film having the xe2x80x9cintermediate layerxe2x80x9d or xe2x80x9ctop coatxe2x80x9d make possible the processes described in these patents.
In each case, to form a more conforming dry film, the protective polyethylene sheet is first peeled away and the exposed surface of the photoimageable composition layer is applied to the surface of the printed circuit board. Using vacuum, heat and mechanical pressure, the dry film is laminated to the surface of the printed circuit board, partially conforming the photoimageable layer thereto. Within about 60 seconds and before substantial cooling of the printed circuit board and dry film has occurred, the cover sheet of the dry film is removed, whereupon the photoimageable composition layer and overlying top coat fully conform to the contours of the printed circuit board and substantially encapsulate the traces and surface contours before conventional processing. Because the cover sheet is removed prior to the final conforming step, better conformance, particularly when applying thin photoimageable composition layers onto boards with closely spaced traces, is achieved. Better resolution is also achievable because the top coat may be directly contacted with artwork for contact printing and because the top coat is much thinner than a cover sheet or support film and is, therefore, much less a deterrent to good resolution than a support film.
In U.S. Pat. No. 4,946,524 (Stumpf et al.), there is disclosed an applicator and process for applying a conforming dry film material to the surface of a printed circuit board allowing for, at the same time, the removal of the protective sheet, subsequent handling of the board with the applied film, and the draw-off of air enclosed between the film and the board. The draw-off of air enclosed between the dry film and the surface of the printed circuit board is facilitated when, before vacuum lamination, the surface of the board is covered with a loose sheet of film. To that end the applicator of U.S. Pat. No. 4,946,524 is operative to tack the dry film to a board at the leading and trailing edges with the intermediate portion of the film loosely applied thereto. The film is tacked to the board as a discrete cut sheet within the confines of the perimeter of the surface of the board. For convenience, a printed circuit board having such loose application of a dry film sheet to the surface or surfaces thereof is referred to hereinafter as being xe2x80x9cprelaminated.xe2x80x9d
In order to adapt the processes described in the preceding patents for continuous automatic operation in an in-line system, there is disclosed in U.S. Pat. No. 5,292,388 (Candore) an automatic conveyorized vacuum laminator apparatus. The apparatus of U.S. Pat. No. 5,292,388 provides an improved and efficient means for automatically conveying and applying vacuum, heat, and mechanical pressure to prelaminated printed circuit boards or substrates and overcomes the difficulties encountered with the utilization of a conventional batch vacuum laminator in an automated in-line system. The automatic conveyorized vacuum laminator is comprised of two main parts, a vacuum laminator and an input roll conveyor for feeding prelaminated circuit boards into the vacuum laminator from the preceding prelaminating equipment. The vacuum laminator, in particular, comprises a vacuum chamber defined by heated upper and lower platens, and an endless belt conveyor disposed between the platens for movement of the printed circuit boards into and out of the vacuum chamber region. The endless belt conveyor, in particular, comprises an endless belt having one section upon which the board can be placed and an aperture therein such that, as the board is moved on the endless belt conveyor into the region of the first vacuum chamber between the upper and lower platens, the aperture is moved into vertical alignment with the lower platen. In operation, the prelaminated circuit board (i.e., having the dry film photoimageable material loosely applied to its surface) to be vacuum laminated is transferred from the input roll conveyor to the endless belt conveyor which moves the board into proper vacuum lamination position between the heated upper and lower platens. Thereafter, the lower platen is raised up through the aperture in the endless belt into sealing engagement with the upper platen in order to capture in the vacuum chamber both the belt and the prelaminated board then resting on the surface of the belt. Next, a vacuum is drawn in the vacuum chamber between the platens to evacuate all air between the dry film and surface of the prelaminated board, followed by application of heat and mechanical pressure to conform the dry film to the board. When the cycle is complete, the lower platen is lowered and the laminated board in conveyed away by the belt conveyor to subsequent processing equipment and the endless belt is returned to its initial position, while the next board to be vacuum laminated arrives for the next vacuum lamination cycle.
Difficulty has been encountered, however, with the operation of the conveyorized vacuum lamination apparatus, as described in U.S. Pat. No. 5,292,388. Particularly, premature tacking of the dry film to the board surface prior to chamber evacuation has been a problem. The problem is particularly prevalent with thin boards (e.g.  less than 0.25 mm.), since they are susceptible to rapid heating. In order to assure complete conformance of the dry film around the circuit traces and substrate surface contours, it is necessary that the loose sheet of dry film prelaminated to the board allow for all air enclosed between it and surface of the printed circuit board to be evacuated before applying heat and mechanical pressure to conform the film to the board. Yet, with the above apparatus, the residual heat given off by the endless belt of the endless belt conveyor just after having completed a prior vacuum lamination cycle has a tendency to cause premature tacking of the film on the next board entering into the vacuum chamber prior to commencement of the vacuum lamination cycle. Premature adhesion prevents air from escaping from certain areas along the film during vacuum lamination, which, in turn, prevents film conformance. In the case of solder masks, lack of film conformance results in lamination defects, such as unwanted puddling caused by premature adhesion. In the case of primary resists, lack of film conformance tends to result in voiding of entire portions of the circuit traces caused by incomplete adhesion, as well as puddling as previously described. The present invention was devised to address this problem.
While there has been some attempt to address this premature tacking problem, a satisfactory answer has yet to be devised. For instance, it has been proposed to process the dry films in conventional batch-oriented vacuum laminating equipment outfitted with removable copper heat shields between the upper and lower platens. The removable heat shields are manually inserted between the upper and lower platens immediately before placement of the board in the vacuum chamber. Evacuation is then commenced with the heat shields serving to insulate the resist from elevated temperatures long enough to be able to remove all of the air between the resist and the board before application of heat and mechanical pressure. However, batchwise processing is highly undesirable because it is entirely too slow for mass production of printed circuit boards and extremely labor intensive.
An object of the invention is, therefore, to provide an improved method of and apparatus for applying under vacuum, heat and mechanical pressure a dry film photoresist or solder mask to prelaminated printed circuit boards or other substrates, thereby to remove all of the air entrapped between the dry film and the surface of the printed circuit board or substrate to assure complete conformance of the dry film around the raised circuit traces and the substrate surface contours.
Another object of the invention is to provide an improved method of and apparatus for vacuum laminating prelaminated printed circuit boards and substrates, which method and apparatus prevent premature tacking of the loosely applied prelaminated dry film to the surface of the printed circuit board or substrate prior to evacuation of all of the air between the dry film and the board or substrate surface.
Still another object of the invention is to provide an improved method of and apparatus for vacuum laminating prelaminated printed circuit boards and substrates which are both operable in an in-line system and in a fully automated continuous manner.
In accomplishing the forgoing and other objectives of the invention, there is provided an improved method of laminating a prelaminated printed circuit board or other substrate which prevents premature tacking of the dry film photoresist-forming layer to the board comprising the following key features: (a) placing a prelaminated board or substrate on one section of a lower moving belt conveyor-for movement into a vacuum chamber region of a dual belt vacuum laminator for the application of vacuum, heat and mechanical pressure thereto, the lower belt conveyor being characterized in the use of an endless belt having two distinct sections upon which the board can be placed, the two sections being so positioned such that, when one section of the lower belt conveyor is moved with the board into the vacuum chamber region, the other section is moved out of said region for cooling and vice versa; (b) indexing a section of an upper belt conveyor spaced above said lower belt conveyor into the vacuum chamber region, the upper belt conveyor being characterized in the use of an endless belt having at least two distinct sections, the at least two sections being so positioned such that, when one section of the upper belt is moved into the vacuum chamber region, at least one other section is moved out of said region for cooling and vice versa; (c) stopping the movement of the lower belt conveyor once the board is moved into proper position in the vacuum chamber of the vacuum laminator; (d) drawing a vacuum in the vacuum chamber for a time sufficient to evacuate substantially all of the air from between the dry film and the surface of the board or substrate and to place the dry film in intimate contact with the surface of the board or substrate, followed by application of sufficient heat to cause the dry film to flow and sufficient mechanical pressure on the board or substrate to thereby force the heated laminate to conform closely to the surface contours of the board or substrate; and, (e) after the vacuum lamination process is complete, conveying the processed board or substrate off of the hot belt section of the lower belt conveyor while loading a new prelaminated board on the other section of the lower belt conveyor which has been allowed to cool outside the vacuum laminator during the previous lamination cycle; (f) and repeating steps (a) through (e) with a new prelaminated board.
The aforesaid steps enable the board being vacuum laminated to be initially disposed in the vacuum chamber only between cool sections of the upper and lower belts. Since the upper and lower belt sections in the chamber are cold, they act as heat shields to prevent the dry film from heating up too fast and prematurely adhering to the board, when exposed to residual heat given off by the vacuum laminator which is still hot from a previous vacuum lamination cycle, before the film can be completely evacuated of entrapped air.
The aforesaid steps are preferably performed in-line and in a continuous automated manner, so that the method can be adapted for use in an fully automated in-line system for manufacturing printed,circuit boards.
In accomplishing these and other objectives of the invention, there is also provided an improved dry film photoresist or solder mask vacuum lamination apparatus comprising the following key features: the provision of a vacuum laminator having two independent (i.e., dual) belt conveyor systems associated therewith, specially a lower belt conveyor and an upper belt conveyor; the lower belt conveyor being operatively positioned for movement of the prelaminated board or substrate into and out of the vacuum laminator for application of vacuum, heat and mechanical pressure to the board and being characterized in its use of an endless belt having two distinct sections upon which the board can be placed, the two sections being so positioned such that, when one section of the lower belt is moved into the vacuum chamber region of the vacuum laminator, the other section is moved out of said region and vice versa, and the upper belt conveyor being spaced above said lower belt conveyor in the vacuum region of the vacuum laminator, and, the upper belt conveyor being characterized in its use of an endless belt having at least two distinct sections that alternate into and out of the vacuum chamber region of said vacuum laminator such that, when one section of the upper belt is moved into the vacuum chamber region, at least one other section is moved out of said region and vice versa; such that in operation as one section of the lower belt upon which the board or substrate to be vacuum laminated is placed is moved into the vacuum chamber region of the vacuum laminator, one section of the upper belt is moved into the vacuum chamber region, and the other sections of the lower and upper belts are moved out of the vacuum chamber for cooling and vice versa.
The aforesaid apparatus is preferably further characterized by the capacity thereof for automated continuous operation. It is also preferable to provide such a continuously operative conveyorized vacuum applicator that is operative, in association with an automated input roll conveyors for feeding prelaminated printed circuit boards or substrates onto the automated conveyor belts, in such a way as to allow at least one board or substrate to be in the vacuum laminator, while the next board or substrate to be vacuum laminated is staged in position on an input roll conveyor ready for the next vacuum lamination cycle to begin. Upon completion of the vacuum lamination cycle, the printed circuit board is automatically conveyed out of the vacuum laminator, and the staged new printed circuit board to be vacuum laminated is conveyed into the first vacuum chamber.
The automatic conveyorized vacuum applicator has particular utility in conveying printed circuit boards and applying vacuum, heat and mechanical pressure to printed circuit boards that have been prelaminated with photoresist or solder mask dry film in accordance with the process described in U.S. Pat. No. 4,946,524 and fabricated in accordance with processes described in U.S. Pat. Nos. 4,889,790, 4,992,354, and 5,164,284.
The conveyorized dry film photoresist or solder mask applicator of the invention is an important component in the total arrangement of an automatic continuous flow of material in in-line processing of dry photoresist or solder mask films requiring vacuum lamination during processing.
The invention provides the means to automate the vacuum application process as an in-line system, while at the same time 1) reducing common lamination defects, such as premature resist adhesion, and 2) substantially eliminating the need to repair or rework finished printed circuit boards.